(1) Field of the Invention
This invention relates to a new and improved process for preparing sulfonated lignins used as dispersants for dyestuffs and more particularly to a process for reducing the viscosity of sulfonated lignins.
(2) The Prior Art
Dyestuff compositions generally comprise a dye cake, i.e., of disperse dyes or vat dyes, and a dispersant. These dyestuff compositions are widely used to color both natural and synthetic fibers. In the dyestuff composition, the dispersant serves three basic functions:
(1) it assists in reducing the dye particle to a fine size,
(2) it maintains a dispersing medium; and
(3) it is used as a diluent.
Dye dispersants are generally one of two major types, sulfonated lignins from the wood pulping industry via the sulfite or kraft processes or napthalene sulfonates from the petroleum industry.
The advantages of employing sulfonated lignins as dispersants in dyestuff compositions is based on their unique physical properties which include good compatibility to many dye systems, outstanding dispersant characteristics at ambient and elevated temperatures and availability. There are, however, a number of disadvantages in the use of sulfonated lignins as dye dispersants. Sulfonated lignins are highly chromophoric and adsorptive in nature towards most fibers and will reduce diazo-type dyes. Their tendency to stabilize foams and to stain along with their high pH and high ash content also add to the undesirable properties of sulfonated lignins as surfactants. A number of technological developments have resulted in new methods and processes to modify sulfonated lignins to reduce the negative aspects of employing such materials as dye dispersants without simultaneously causing any major adverse effects upon those properties which render sulfonated lignins desirable as dyestuff dispersants. By way of illustration the following patents are cited.
U.S. Pat. No. 4,025,303 to G. Prazak describes a sulfonated lignin modified by reacting it with a water-soluble magnesium salt resulting in a sulfonated lignin dye dispersant having retarding properties that are capable of providing the necessary leveling in a multi-color dye process. U.S. Pat. No. 4,001,202 to P. D. Dilling and G. Prazak describes a process for modifying a sulfonated lignin useful as a dye dispersant by reacting such lignin with an epihalohydrin to eliminate fiber staining. Additional examples of reacting or modifying lignin to make them more suitable as dye dispersants include U.S. Pat. No. 4,184,845 to S. Y. Lin; U.S. Pat. No. 4,131,564 to P. Dilling; U.S. Pat. No. 3,156,520 to L. A. Baisdell; U.S. Pat. No. 3,094,515 to K. F. Keirstead et al; U.S. Pat. No. 3,726,850 to Detroit; U.S. Pat. No. 2,680,113 to E. Adler et al.; and U.S. Pat. No. 3,769,272 to Hintz. The art cited is meant to show the state of the art and is not intended to be all inclusive of lignin modifications.
In recent years, the rheological properties of the sulfonated lignin especially at the lower pH's, has become an area of concern. The Imperial Chemical Industries (ICI) in England, for example, has set viscosity standards for dispersants not to exceed 100 centipoises at pH equal to 8 and 25 percent solids concentration. One of the sulfonated lignins manufactured and sold by Westvaco Corporation under the trademark REAX.RTM. (85A) exhibits a viscosity of greater than 400,000 centipoises at pH of 9 with occasional solidification occurring at even higher pH's.
In the past, corrective measures for the negative viscosity behavior of sulfonated lignins have been made by the introduction of additional solubilizing groups into the lignin polymer network by means of increasing the sulfonation of the lignin. In general, the use of such solubilizing groups results in a more hydrophilic sulfonated lignin causing a reduction in the adsorption of the material onto a dyestuff particle as well as a noticeable loss in the heat stability of the lignin product.
It is the general object of this invention to provide a process whereby sulfonated lignins or lignosulfonates may be treated to improve their usefulness as dye dispersants.
Another object of this invention is to provide a process for reducing the viscosity of sulfonated lignins or lignosulfonates.
Other objects, features and advantages of this invention will be seen in the following detailed description of the invention.